The central hole within a vertebra is termed the spinal canal and it houses and protects the spinal cord and/or spinal nerves. Whether from degenerative disease, traumatic disruption, infection or neoplastic invasion, the spinal canal may become narrowed over one or more vertebral levels and lead to compression of the indwelling neural tissues. Narrowing of the spinal canal is termed spinal stenosis and this condition can produce significant pain, neurologic dysfunction and disability. In addition, mal-alignment of adjacent spinal vertebrae can further narrow the spinal canal and cause additional pain and disability.
The current surgical treatment of spinal stenosis is decompression of the neural tissues by removal of the bone and ligament elements that produce nerve compression. Laminectomy, the removal of the lamina segment of the vertebrae, is the most common way to achieve decompression of the spinal canal and hundreds of thousands of patients undergo this operation every year in the United States alone. Although the operation provides nerve decompression, it also has negative and permanent side-effects upon the spinal segments.
Laminectomy removes one side of the nerve's natural bony housing and leaves the posterior aspect of the neural tissues exposed and unprotected. The exposed nerves are vulnerable to injury and this vulnerability is especially problematic if future surgery is required. At the time of re-operation, the nerves have no posterior bony covering that can be used as a marker of nerve location. In addition, the dural sac that naturally encases the nerves will invariably scar onto the surrounding soft tissues thereby obscuring the tissue layers and making the exact position of the nerves unknown to the operating surgeon. This makes the dural sac and contained nerves particularly vulnerable to inadvertent injury during subsequent surgery and a dural injury rate of 10-20% rate has been reported at the time of re-operation. These dural violations can lead to permanent nerve injury, disability and chronic pain.
Laminectomy produces a defect in the bone and ligament structures that ensure the longitudinal alignment of adjacent vertebra and weakens the structural integrity of the spine. Many patents that undergo laminectomy will subsequently develop spinal instability and disabling pain. To treat the instability, various devices have been developed to support the spine. While some of these devices permit motion, others promote fusion and complete immobilization of the unstable spinal segments. Regardless of the specifics of design or function, these devices are anchored onto the vertebral bodies with bone screws or similar fasteners and will require a stable attachment platform onto which they may be affixed.
The stability of the attachment platform is critical to the proper function of the implant and those devices that are poorly anchored to the underlying bone will inevitably loosen with repeated vertebral movement. Solid device attachment is especially important in the implantation of devices that preserve vertebral motion. While fusion devices bear load until the bone has fused, motion preservation implants must provide indefinite support of the vertebral movement. As motion preservation implants are used with increasing frequency, there is a growing need for an improved method of attachment onto the underlying vertebral bone.